It is known that a light-sensitive material using a photopolymerizable composition is sensitized by heating after it is exposed. This sensitization by heating usually occurs in the solid phase, increasing the sensitivity to from several to several ten times the original value. An advantage of increasing the sensitivity by heating resides in that exposure time can be shortened, increasing an operation efficiency and at the same time, making it possible to save energy. When the sensitivity of the light-sensitive material is markedly increased, it can be subjected to enlarged exposure and further, it is possible to apply scanning exposure using laser.
Another advantage is that it is possible to utilize a photopolymerization initiator which is of low sensitivity, has good stability, and is inexpensive. This permits the production of a light-sensitive material which has good stability and is inexpensive.
Incorporation, on the other hand, of such a heating step into the plate making process increases the number of steps and makes the process complicated. In particular, problems arise in processing a large number of light-sensitive materials. It has been found that a definite time interval between the heating and the developing steps of a light-sensitive material is important. It has also been found that sensitivity is quite different between when the light-sensitive material is developed immediately after it is heated and when a number of light-sensitive materials are heated and, after heating of all the materials is completed, they are developed.